If particularly experienced players are at the table, you can ask them to take on this handicap to equalize play. You play without the volcano chip; instead, every handicapped player receives 1 minus point for every active volcano.
The players without a handicap receive no minus points for volcanos.
Inactive volcanos bring no minus points. You can increase the handicap further by making the inactive volcanos also carry 1 minus point each.
Back Side of the World Board
The back side of the world board offers you a further opportunity for variation.
You can play any level of difficulty, including the solo game, with it. Landscapes which include the edge of the board count as enclosed on that edge. (As usual, water landscapes do not count. They also do not count as a lake if they include the water edge).
When the timer is set and before a player places his first tile he is allowed to choose for himself which side of the world board he uses.
Place the timer so that no one can see the time remaining.
If you've got some practice, simply shorten the time to, e.g., 4 minutes or if a player quits the turn ends immediately for all. For that it is only allowed to have not more than 3 empty fields.
At the beginning of a turn, the timer will not be started. The first player who quits sets the timer for 1 minute.
Random Additional Tasks
Don't place the additional tasks in the middle. Distribute them instead randomly to the players. They have to be of course fulfilled otherwise you get the usual minus points.
It is not allowed to flip landscape tiles at anytime. Means: after placing a tile on the world board the top side of the tile is still the same as before in the middle.
Sologame for two
Play according to the solo game rules. The Volcano Chip applies for both players. Additionally play with the two Bonus Chips. As usually the player wins with the least minus point (the Bonus Chip gets charged positively) . Play a 7 minutes time limit.
If children younger than the recommended age are playing, use the beginner game with a longer time and simplify the scoring.